Subject: CVE-2021-47243: sch_cake: Fix out of bounds when parsing TCP options and header

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

sch_cake: Fix out of bounds when parsing TCP options and header

The TCP option parser in cake qdisc (cake_get_tcpopt and
cake_tcph_may_drop) could read one byte out of bounds. When the length
is 1, the execution flow gets into the loop, reads one byte of the
opcode, and if the opcode is neither TCPOPT_EOL nor TCPOPT_NOP, it reads
one more byte, which exceeds the length of 1.

This fix is inspired by commit 9609dad263f8 ("ipv4: tcp_input: fix stack
out of bounds when parsing TCP options.").

v2 changes:

Added doff validation in cake_get_tcphdr to avoid parsing garbage as TCP
header. Although it wasn't strictly an out-of-bounds access (memory was
allocated), garbage values could be read where CAKE expected the TCP
header if doff was smaller than 5.

The Linux kernel CVE team has assigned CVE-2021-47243 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 4.19 with commit 8b7138814f29 and fixed in 4.19.196 with commit 595897ef118d
	Issue introduced in 4.19 with commit 8b7138814f29 and fixed in 5.4.128 with commit 4cefa061fc63
	Issue introduced in 4.19 with commit 8b7138814f29 and fixed in 5.10.46 with commit 3b491dd593d5
	Issue introduced in 4.19 with commit 8b7138814f29 and fixed in 5.12.13 with commit 3371392c60e2
	Issue introduced in 4.19 with commit 8b7138814f29 and fixed in 5.13 with commit ba91c49dedbd

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-47243
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	net/sched/sch_cake.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/595897ef118d6fe66690c4fc5b572028c9da95b7
	https://git.kernel.org/stable/c/4cefa061fc63f4d2dff5ab4083f43857cd7a2335
	https://git.kernel.org/stable/c/3b491dd593d582ceeb27aa617600712a6bd14246
	https://git.kernel.org/stable/c/3371392c60e2685af30bd4547badd880f5df2b3f
	https://git.kernel.org/stable/c/ba91c49dedbde758ba0b72f57ac90b06ddf8e548